1. Field of the Invention
The present invention relates to a radiation detector including at least one switching element. The radiation detector may be used for detecting medical images. The present invention further relates to a medical image apparatus for detecting medical images using the radiation detector.
2. Description of the Related Art
It is known that a radiation detector in a medical image apparatus, such as an X-ray diagnosis apparatus, uses, for example, a thin film transistor (hereinafter referred to as TFT) as a switching element for each pixel of the radiation detector. The radiation detector is usually formed by repeating processes for forming a thin film on one surface of a glass substrate, patterning the thin film by etching, forming another thin film to overlap the etched pattern, and patterning the latter thin film.
FIG. 1 is a block diagram showing a radiation detector according to a prior art of the present invention. The radiation detector has a plurality of pixels 300 arrayed in a matrix. Each pixel 300 comprises a signal-read TFT 301, a photoelectric conversion element 302, and a capacitor 303. The photoelectric conversion element 302 is formed of selenium and can directly convert incoming radiation, such as an X-ray, into a charge. The capacitor 303 stores the charge. The charge stored in the capacitor 303 of each pixel 300 is read out to an integrating circuit 311 through the signal-read TFT 301 and a signal line 305.
As shown in FIG. 1, a gate driver 308 controlled by a timing controller 332 turns on/off signal-read TFTs 301 by charging a predetermined potential on vertical select lines 306. Upon switching the signal-read TFTs 301 between ON and OFF, a charge injection is caused due to an apparent capacitance (a parasitic capacitance or a stray capacitance) (hereinafter referred to as a parasitic capacitance) each of which is generated between a gate of the signal-read TFT 301 and the signal line 305. The charge injected may be determined by the parasitic capacitance and a potential difference between a voltage at the time of ON of the signal-read TFT 301 and a voltage at the time of OFF of the signal-read TFT 301. The charge injected is discharged to the signal line 305 and may be read out to the integrating circuit 311 with the charge stored in the capacitor 303. Accordingly, the charge injected influences a charge to be stored in a capacitor 310. Particularly, in case of a fluoroscopy, an incoming signal to be detected is usually small. To detect such a small signal accurately, a capacitance of the capacitor 310 is usually set small. Therefore, due to the charge injected in the capacitor 310, a dynamic range of an amplifier 307 for reading out detected signals (charges discharged from each pixel 300) is narrowed. Further, in some cases, it may cause saturation in the amplifier 307.
As disclosed, for example, in Japanese Patent Application Publication (Kokai) No. 2001-56382, providing dummy pixels 309, the influence by the charge injection may be reduced to a certain extent. Each dummy pixel 309 comprises a TFT 361 and a capacitor 362. A gate driver 330 controlled by the controller 332 turns on/off the TFT 361 of each dummy pixel 309 by charging a predetermined potential on a control line 363 in a reversed phase manner to the switching of the signal-read TFT 301. The predetermined potential charged on the control line 363 may be the same as the potential difference between ON and OFF of the switching of the signal-read TFT 301. Accordingly, the charge injected may be balanced out by the predetermined potential charge on the control line 363. An output of each integrating circuit 311 is supplied to a multiplexer 320.
Although the above technique may be a great improvement for X-ray detection, a prior art radiation detector is still subject to narrowing of its dynamic range. When, for example, charges are read out from the pixel 300 at a high speed, such as 30 frames per second, a difference appears between a rounding of pulses for driving the signal-read TFT 301 and a rounding of pulses for driving the TFT 361. Further, a fluctuation in amounts of the charge injection appears over the array of pixels. Accordingly, a dynamic range of the amplifier 307 can become small. This may disturb reading out detection signals (charges) obtained in the photoelectric conversion element 302 and stored in the pixel 300 in a preferable manner.
Particularly, when the capacitor 310 has only a small capacitance, the amplifier 307 has a possibility of losing its dynamic range even only due to a fluctuation in offsets resulting from the fluctuation in amounts of the charge injection over the array of pixels.